This invention relates to a reaction injection molding process for preparing molded urethane-based products containing rigid fibers having a diameter of from about 5 to about 10 micrometers as a reinforcing filler.
The use of fillers in polyurethanes is well known. For example, structural polyurethanes manufactured by reaction injection molding ("RIM") typically contain one or more reinforcing fillers. The selection of the type and quantity of reinforcing fillers in polyurethanes prepared by the RIM process is in general based on the desired performance criteria. The addition of fillers to structural parts provides benefits for a number of reasons, including improved modulus, altered thermal properties such as sag, shrink, and thermal expansion. Fillers are typically added to the isocyanate-reactive component but can sometimes also be added to the isocyanate component as well.
Milled glass fiber has been widely used as a filler for polyurethanes. E.g., U.S. Pat. Nos. 4,381,352, 4,680,214, and 4,861,803. The standard milled glass fiber used in the polyurethane industry has a diameter of about 16 micrometers and nominal lengths referred to as "1/32 inch", "1/16 inch", "1/8 inch", or "1/4 inch" (preferably "1/16 inch"). E.g., S. H. Metzger, Jr. and K. Seel, "High Modulus RIM Elastomers for Automotive Exterior Body Panels" in J. Cell. Plastics, 268-273 (1981); see also U.S. Pat. No. 4,381,352. The term "nominal length" as used in the glass industry does not refer to average lengths for a given fiber sample but is instead related to the size of a sieve through which the samples can pass. Within the glass industry, the length of a glass fiber can be expressed in various ways, such as average fiber length or as bulk density. For example, the average length of a "1/16-inch" milled glass fiber is about 0.006 inches (0.15 mm), with the range being from about 0.001 inches (0.025 mm) to about 0.04 inches (1.0 mm), and the bulk density of such a fiber is about 0.500 g/cm.sup.3.
It has now been found that rigid fiber, particularly milled glass fiber, having a diameter ranging from 5 to 10 micrometers provides unexpected advantages when used as a filler for urethane-based products. For example, reinforced polyurethanes prepared using milled glass fibers having a diameter of about 5 to about 10 micrometers exhibit physical properties that are equal to or better than reinforced polyurethanes containing more than twice the quantity of 16 micrometer glass fibers.